Improved phi reachability computation a bit.

src/hydrogen.cc

Index: src/hydrogen.cc
===================================================================
--- src/hydrogen.cc	(revision 9184)
+++ src/hydrogen.cc	(working copy)
@@ -1675,7 +1675,9 @@
   bool change = true;
   while (change) {
     change = false;
-    for (int i = 0; i < phi_count; ++i) {
+    // We normally have far more "forward edges" than "backward edges",
+    // so we terminate faster when we walk backwards.
+    for (int i = phi_count - 1; i >= 0; --i) {
       HPhi* phi = phi_list->at(i);
       for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
         HValue* use = it.value();